Conventionally, a tire-pressure monitor system (Tire Pressure Monitoring System: TPMS) which informs a driver of tire-pressure information has been known. The tire-pressure monitor system comprises a tire-pressure sensor unit (referred to as a wheel sensor) which detects a tire pressure for each wheel, and is configured to transmit tire-pressure information as a radio signal from this wheel sensor and receive this radio signal by a vehicle body side device to acquire the tire-pressure information. When judged that the tire pressure has been decreased based on the received tire-pressure information, the vehicle body side device displays that on an annunciator to notify it to a driver.
The vehicle body side device receives not only the radio signal transmitted from the wheel sensor disposed in the wheel of the vehicle (self-vehicle), but also the radio signal transmitted from the wheel sensor disposed in many and unspecified vehicles. For this reason, in order to distinguish from which wheel sensor the radio signal that the vehicle body side device receives is transmitted; the radio signal contains a sensor ID which is discrimination information unique to a wheel sensor, in addition to the tire-pressure information.
The sensor ID of the wheel sensor disposed in the wheel of the vehicle is beforehand registered into a non-volatile memory of the vehicle body side device. Therefore, the vehicle body side device is configured to refer to the sensor ID memorized in the non-volatile memory and acquire the tire-pressure information included in the radio signal specified by this sensor ID as the tire-pressure information of the wheel of the vehicle (self-vehicle).
In the Patent Document 1 (PTL1), a device which registers a sensor ID of a wheel sensor disposed in a wheel of the vehicle (self-vehicle) according to a wheel location is proposed. In this device, a user gives an impact to tires of right-and-left front-and-rear wheels of a vehicle in an order described in documented work instructions. The wheel sensor disposed in each wheel contains an acceleration sensor, and transmits own sensor ID to the vehicle body side device when this acceleration sensor detects the impact acts on the tire. Therefore, the sensor ID transmitted from each wheel sensor is transmitted to the vehicle body side device in the order in which the user gave an impact to the tire, i.e., the order described in the documented work instructions. Based on this order in which the sensor IDs are received, the vehicle body side device acquires the correspondence relation between the sensors ID and wheel locations, and memorizes the correspondence relation into the non-volatile memory. Therefore, in this device, rewriting of the sensor IDs is performed by a users manual operation.
Moreover, a technology for distinguishing a sensor ID of the wheel sensor of a self-vehicle out of sensor IDs of the radio signals transmitted from many and unspecified wheel sensors and automatically registering the same has been also known. For instance, in the Patent Document 2 (PTL2), a device which memorizes all the sensor IDs of radio signals received until a predetermined time has passed since a radio signal transmitted from a wheel sensor for the first time (this period is referred to as an entry period) and thereafter searches for the sensor ID of the wheel sensor of the vehicle among the memorized sensor IDs is proposed. In this device, a sensor ID presumed to be assigned to a wheel sensor of a vehicle other than the vehicle (self-vehicle) is eliminated from the sensor ID memorized as registration candidates during the entry period. Then, when the number of remaining sensor IDs becomes equal to the number of the wheels of the self-vehicle, the sensor IDs are registered as the sensor IDs of the wheel sensors of the self-vehicle.